The superstar surprised the band after three songs, sharing a microphone with singer Matt Ramsey on the Ramsey and Brad Tursi co-written "Save It for a Rainy Day," a hit from Chesney's The Big Revival album. Before wondering aloud why Old Dominion didn't save it for themselves, the reclusive singer recalled that the group opening his stadium tour several years ago, playing before the show technically started. Now they have six hit radio singles.

Brand new songs called "Smooth Sailing" and "Make It Sweet" ("Life is short, so make it sweet," Ramsey sings) were mixed between hits like "Hotel Key," Song for Another Time," "Written in the Sand" and "No Such Thing as a Broken Heart," but it was the guest appearances that the Ryman crowd will remember. Chesney was first, but two songs later Old D tour opener Ray came to the stage to sing his new single, the Ramsey and Trevor Rosen co-written "One That Got Away."

Old Dominion (including Geoff Sprung and Whit Sellers) formed a decade ago, but Ramsey and Rosen recalled moving to Nashville in 2003 to begin songwriting careers. The band was something of a side project — something they've said they never imagined working out to the extent that it has (two albums, four Billboard No. 1 hits, perennial Vocal Group of the Year nominations). Since 2011 they've individually written some of the decade's most recognizable hits for artists like Hunt, Chesney, Dierks Bentley,Tyler Farr,Blake Shelton and Luke Bryan. This concert — which raised $40,000 for the Ryan Seacrest Foundation and the Opry Trust Fund — subtly paid homage to this other, arguably more impressive talent.

Ballerini walked out singing OD's "Written in the Sand" like it was recorded as a duet before telling a story with Rosen about how they worked on "I Hate Love Songs Together." Her playfully delightful performance of this hit segued neatly into the group's "Crazy, Beautiful Sexy." A few minutes later, Hunt provided the biggest surprise of the night. Even if you were aware of how important songs penned by Old D members were to his Montevallo album, you probably didn't realize that they all kind of grew up in Nashville together. The guys hugged and talked to each other like it was a Hall of Fame reunion before playing sincere versions of "Make You Miss Me" and "Ex to See," both co-written by Ramsey.

The 90-minute-plus performance never dipped long enough for fans to feel comfortable leaving to use the bathroom or to buy another beer. They found themselves watching the side of the stage as frequently as the actual stage in hopes of sneaking a peek at the next guest artist.

While Old Dominion did say they're working on new music, they didn't indicate which song would be their next single. Look for them back on the road in October.